1. Field of Invention
This invention relates to gravure engraving and, more particularly, to an engraving system and method for providing an improved engraved image by utilizing digital toolpath planning, digital tuning and closed loop feedback control.
2. Description of the Related Art
The process of traditional electromechanical engraving consists of cutting small areas or cells of copper from a surface of a copper cylinder. In the traditional process, the cells are formed by superimposing a screening signal and an image signal. The screening signal is an AC sinusoidal waveform and the image signal is added to it to cause a cutting tool or stylus to move or oscillate to engrave the cells.
In the past, most efforts to improve the quality of an engraved image were focused upon modifications to the image signal. Moreover, edge enhancement, selective linearization, speeding up tuning of the engraving head, and pulse modulation were also used to modify the image signal to improve the engraving quality. One or more of these concepts in prior art engraving systems shown in U.S. Pat. Nos. 5,438,422; 5,424,845; 5,621,533; 5,329,215; 5,416,597; 5,424,846; 5,402,246; 5,454,306; 5,475,914; 5,555,473; 5,440,398; 5,493,939; 4,357,633; 4,438,460; 4,450,486; 4,451,856; 4,500,929; 5,492,057; 5,029,011; 5,519,502; 5,583,647; 5,491,559; 5,422,958; 5,293,426; and 5,617,217 disclose apparatus and methods of the general type used in processes in the past.
Thus, it should be apparent that attempts made to improve engraving quality focused upon, for example, modification of the image signal.
Similarly, U.S. Pat. Nos. 4,245,260 and 5,519,502 illustrate techniques for linearizing and shifting of engraved areas to provide a higher quality engraving. For example, U.S. Pat. No. 5,245,260 discloses techniques for modifying a screening signal in order to control the placement of the cell.
What is needed, therefore, is a system and method which synthesizes the image signal and the screening function as a reference wave form which represents the desired stylus toolpath, thereby reducing or eliminating the need to modify the image and screening signals altogether.
Another problem with the approaches of the past is that the screening and layout of the cells were directly dependent upon the AC sinusoidal waveform, thereby eliminating the flexibility of the engraving quality because prior screening waveforms have consisted of constant AC sinusoids which were difficult to modify sensitive to the image to be engraved. What is also needed, therefore, is a simple, yet effective, method and apparatus for transforming data associated with an image to be engraved into a signal which can be synthesized as an arbitrary toolpath derived directly from the image data.
Moreover, in the past, the mechanical structure of an engraving head exhibited resonance characteristics such that at approximately 2 KHz and 5.5 KHz, the engraving head produced increased output. In order to eliminate these undesired resonances, notch filters were added so that the frequency response of the engraving head was more predictable. Due to the nature of the filter algorithms for the notch filters, a generally smooth, but decreasing, response was achieved. The aforementioned U.S. Pat. Nos. 4,357,633; 4,438,460; 4,450,486; 4,500,929 disclose engraving apparatus and methods of the general type which utilize filtering processes of the past.
Unfortunately, the traditional filtering techniques generally produced an engraving head frequency response which did not carry enough amplitude at higher frequencies, such as frequencies in excess of 5.5 KHz. In addition, the techniques of the past did not permit a user to interactively and easily change the characteristics, such as filter coefficients, of the filters in order to manipulate or tune the engraving head as desired. In the past, the filter characteristics were subject to electronic component errors and could not be moved from one circuit board to another with the required precision.
What is needed, therefore, is a system and method which characterizes the engraving head digitally such that the tuning filter characteristics may be stored separate from the tuning circuitry. The digital filter characteristics may then be archived and in the event of circuit failures loaded into the memory of a replacement circuit with sufficient precision to yield the desired engraving head response characteristic.
In the past, correcting errors or calibrating the engraving head was performed in various manners using various techniques, such as the techniques shown in U.S. Pat. Nos. 5,029,011; 5,293,426; 5,416,597; 5,422,958; 5,438,422; 5,440,398. For example, in some calibrating processes of the past, various characteristics of an individual cell in a test cut were measured and compared to a desired characteristic. If the difference between the desired characteristic and actual characteristic were greater than a predetermined tolerance, then the engraving head was adjusted to account therefor. Typically, the prior art processes utilized optics, strobing, and focusing equipment to achieve such measurements and correction. What is needed is a tool velocity and position sensing system and feedback system and method which is simple in design, relatively inexpensive and easy to implement.
What is needed, therefore, is a method and system for affectively manipulating the placement of an engraved area directly from the image data without either using or manipulating the screening signal.
In the past, the resultant head response characteristic after the lengthy tuning process was subject to the tuning operator's subjective judgement as to tune quality. This led to variations from head to head and difficulty in matching engraving image quality between different engravers and also between heads on multiple headed publications engravers.
What is needed, therefore, is a method and system for automating the tuning process thus removing the variations in engraving head response characteristics. Such a system will improve tuning repeatability in addition to shortening the time required to tune an engraving head.